The present invention relates to a piston for a hydraulic motor having radial pistons, said piston comprising a body having a guiding and sealing surface, a base, and a top, which top is provided with a cradle-shaped recess.
The cradle-shaped recess in the top of the piston serves to receive a roller or wheel designed to roll on the cam of the radial-piston hydraulic motor. In the meaning of the present invention, a cradle-shaped recess has a concave surface that is substantially in the shape of a fraction of a cylinder.
While a radial-piston motor is operating, the cylinder block and the cam of said motor move in rotation relative to each other, and the pistons move radially in reciprocating motion inside the cylinders of the cylinder block, their above-mentioned rollers rolling on the cam.
During this reciprocating motion, the guiding and sealing surface, which is the side surface of the piston (generally in the shape of a cylinder having a base that is circular or of some other shape), co-operates in leaktight manner against the inside surface of the cylinder in which the piston is received. The cylinder block and the cam moving in rotation relative to each other, the piston is subject to tilt forces that are compensated by the guiding due to the mutual co-operation of the two above-mentioned surfaces. The contact pressures between said surfaces can then be relatively high, and it is desirable to attempt to reduce them. In addition, the friction between the guiding surfaces of the pistons and the inside surfaces of the cylinders causes local heating which, if it becomes excessive, can cause the piston to seize. The fluid present in the cylinder, between the end wall of the cylinder and the base of the piston, limits the heating, in the regions in the immediate vicinity of the base of the piston. Unfortunately, in the portion of the guiding and sealing surface that is remote from said base, the cooling is not performed so well, and the risks of seizure can be high, in particular in the middle regions of the guiding and sealing surfaces.
FR 2 648 512 discloses a hydraulic machine having radial pistons, in which machine each piston has a blind cylindrical hole at its base, centered on its axis of translation, the function of that hole not being mentioned.
An object of the invention is to remedy the above-mentioned drawbacks, with a piston that, in addition, can be manufactured simply and inexpensively.
This object is achieved by the fact that the base is provided with an additional cradle-shaped recess whose concave surface faces in the direction opposite from the direction in which the concave surface of the recess in the top faces.
The recess has a concave surface that is substantially in the shape of a fraction of a cylinder, and that is of area larger than the area of the flat end wall of the piston, so that the area of heat exchange with the fluid present in the cylinder is increased, thereby facilitating cooling of the piston. Another reason why this cooling is more effective than the cooling in the prior art is that the fluid present in the recess is at a radial level that is offset outwards relative to the inner end, thereby making it possible to cool zones of the guiding and sealing surface that are somewhat remote from said inside end.
It should also be noted that the presence of the additional recess causes material to be removed from the base of the piston, thereby naturally enabling the piston to be lightened.
Compared with the blind holes of FR 2 648 512, the additional cradle-shaped recess, which is open at its ends situated in the guiding and sealing surface, enables the structure of the piston to be further lightened, enables the flexibility of the portions of the base of the piston that flank the additional recess to be further increased, and enables cooling to be enhanced, by increasing the area of contact between the cylinder and the fluid present in said cylinder under the base of the piston.
Advantageously, the two cradle-shaped recesses extend parallel to each other.
As indicated above, the pistons are subjected to tilt forces relative to their axes of translation, while the cylinder block and the cam are rotating relative to each other. The tilt forces are applied in planes perpendicular to the axis of relative rotation of the cylinder block and of the cam. The cradle-shaped recesses in the tops of the pistons extend parallel to that axis so as to enable the rollers present in said recesses to roll against the cam. In this situation, when the two recesses of a piston extend parallel to each other, the additional recess is also parallel to the axis of relative rotation of the cylinder block and of the cam. Because of the presence of the additional recess, the portions of the guiding and sealing surface that flank the recess present slight flexibility that enables them to deform in planes perpendicular to the axis of the recess. It is thus precisely in the planes in which the tilt forces are high that said portions of the guiding and sealing surface can deform slightly and can thus increase the mutually contacting surface areas. This makes it possible to limit the negative impact of the tilt forces while reducing the contact pressure and while avoiding excessive localized friction.
The invention also provides a method of manufacturing pistons for radial-piston motors from a cylindrical bar having a longitudinal axis, each of said pistons comprising a body having a guiding and sealing surface, a base, and a top, which top is provided with a cradle-shaped recess.
In known methods, each piston is obtained from a billet, e.g. by cutting up a cylindrical bar into segments. Each billet is drilled so as to form the cradle-shaped recess in the top of the corresponding piston, which recess serves to receive the roller of the piston. In practice, a cylindrical hole is drilled in an end portion of each billet, and, optionally, in order to form a stop surface inside the recess in the top of the piston, that hole is modified by means of a tool of the broaching tool type. In particular, said stop surface serves to retain a cradle-shaped journal-bearing lining against the bottom of the recess, against which lining the roller of the piston rolls, which roller is also retained in the recess. Then, that end portion is cut through the hole so that a portion of the hole, which portion defines the cradle-shaped recess, forms the top of the piston, while that end of the billet in which the other portion of the hole was machined is discarded. That results in considerable wastage of material, it being possible for such wastage to be as high as about 50%.
An object of the invention is to improve that state of the art by proposing a method that makes it possible to achieve higher manufacturing rates and that limits wastage of material.
This object is achieved by the fact that the method consists in machining cylindrical transverse holes at regular intervals in the bar, and, in order to form two adjacent pistons, in cutting up the bar on a transverse cutting-up plane passing through a hole, so that a first portion of said hole forms a cradle-shaped recess in the base of the first one of the two adjacent pistons and so that another portion of said hole forms a cradle-shaped recess in the top of the second one of the two adjacent pistons.
With the method of the invention, the holes are machined directly in the bar, prior to cutting up said bar, and they can thus be formed automatically under excellent conditions. In particular, it is easier to hold the bar correctly over a long length, than to hold the individual billets as in the prior art, while performing the hole-forming operations. In addition, with the invention, a very significant saving in material is achieved because no hole is “wasted”, each hole having a first portion that serves to form the recess in one piston, and a second portion that serves to form the recess in the adjacent piston.
If machining (in particular broaching) is necessary for forming stop surfaces in the recesses in the tops of the pistons, such machining is performed in the bar that is provided with the series of holes, before said bar is cut up.
In addition, each of the pistons obtained by means of this method has two recesses, thereby as indicated above, offering major advantages as regards limiting contact pressures, friction, and weight, and improving cooling of the pistons equipping a radial-piston hydraulic motor.